How far out to place motor from platter

It's your call. For minimum belt creep, the platter and motor pulley should be as close as possible, so that the circumference of the platter is in contact with as much of the belt as is possible. For other reasons, you might want them to be far apart.

note that your speed may change based on how tight the belt is, either due to belt stretch changing the diameter or excess loading on the motor bearing increasing friction. Obviously, both motors should be the same distance away from the platter.

Manitunc, OP asked about distance. Good point to say that both motors should be the equidistant from the spindle. Along those lines, and altho the OP did not ask, the best way to use two motors would be to line them up on opposite sides of the platter. In other words, 180 degrees apart. That way, the belt or strings are "pulling" on the platter in opposite directions, and those side forces should cancel, assuming tension is equal on both strings. That's the only good thing about using more than one motor, IMO.

Thank you for the advice. I will move them opposite

>>> For minimum belt creep, the platter and motor pulley should be as close as possible, so that the circumference of the platter is in contact with as much of the belt as is possible.

This overlooks the other (and more significant) half of the belt contact equation. It's true that placing a smaller circle closer to a larger circle increases the arc segment between the two tangent points around the larger circle, but it correspondengly reduces the arc segment between the two tangent points around the smaller circle. IOW, moving the motor closer INCREASES the belt contact patch around the platter (as Lewm said) but it also DECREASES the belt contact patch around the motor capstan.

As the contact patch around the smaller capstan will always be shorter than the contact patch around the larger platter, the capstan is where belt slippage is most likely to occur. Therefore, your setup shoudl be optimized to lengthen the belt patch around the motor capstan. This is accomplished by moving the motor and platter away from each other. How far? Far enough to optimize the benefit without incurring detriments from belt sag or elasticity.

Lew's argument to place the motors opposing each other to counterbalance tension on the platter bearing has merit, but it's not the only placement consideration. Motor orientation also matters. Assuming the motors are identical and running in synch, orienting them so that they point the same way would amplify any cogging effects (times 2, with 2 motors). Not a good thing.

One advantage of multiple motors is that, if synched and properly oriented, each can mitigate the cogging effects of the other. To do this you must orient them precisely out of phase with each other. How to do that? Find out how many poles the motors have. Lets say it's 12. Divide 360 by 12 (30), then divide that by 2 (15). Orient the motors such that one is twisted 15 degrees clockwise (or counter-clockwise) compared to the other.

Agree with Manitunc that proper belt tension is critical, particularly with a non-elastic belt material. That's what matters with regard to distance. It's not obvious to me, however, that the two motors need be equidistant. Aside from visual aesthetics and questions of available space, what's the benefit? One could even argue that differing distances could reduce the probability of simultaneous belt slippage events, since the contact patches of each setup would be different.

Agree with Lew that one good motor is a better solution than any greater number of flawed motors. The best you can hope for with multiple motors is that their flaws will somewhat cancel each other (as with the anti-cogging orientation described above). Better not to have the flaws in the first place. A non-cogging motor will always have less cogging than two cogging motors, however perfectly arranged. One motor is quieter than two. Etc...

Manitunc, assuming stretching the belt does not change the angular velocity of the motor it will not change the platter "speed" as this depends on the motor's rotation and the ratio of the platters diameter to the pulley's diameter. Changing the length of the belt by stretching does not affect the "speed". Changing the tension can affect the sound though. I have a Nott. Hyperspace whose motor shaft bearing had become "hogged out". Increasing the belt tension by moving the motor further from the platter made the sound unlistenable. Moving it in as close as possible and still maintain rotation made things bearable. Needless to say, a new motor improved things a lot.

John,
Stretching the belt necessarily make it thinner, and therefore, the radius of each pulley changes as the ratio is determined by the radius to the center of the belt at each end. And thats assuming that the increased drag from the side loading of the motor and platter bearing doesnt also slow the platter down. I know on my Transrotor Fat Bob, I can hear it slow down as I pull the motor farther away. There is a point of perfect tension, and it is different on each table/motor/platter/belt

Doug, Your comments are a perfect rationale for why I have become a devotee of idlers and direct-drives. You make a good point about the trade-off associated with having the motor pulley close as possible to the platter, but are we conflating belt "creep" with belt "slip"? I had not thought of that (mostly because I only worry about belt creep after the third slice of pizza). I was actually repeating a statement once made by Mark Kelly, or at least I thought I was. If the idea is flawed, then I probably misinterpreted Mark. There are two better solutions to belt creep. One is using a capstan-like device to keep the belt close to BOTH the pulley and the platter. (One commercial turntable does that, I think.) The other is to use two platters, one driving the other, with the belt wrapped in a way described by Mark and maybe also implemented by RS Labs, such that it is in contact with most of the circumference of both the driver and the driven platters.

I can only imagine one reason why the motors must be equidistant. First, it would seem obvious that we want the two motors to be as identical as possible in all parameters. If so, then it follows that we want their drive pulleys to be rotating at identical speeds, or as identical as possible. Therefore the belts need to be of the identical circumference. From that, does it not follow they would be equidistant from the spindle?

Third try. My last two posts never got posted. In the above post, I misspoke; the 2-platter solution to belt creep is commercially implemented in the 47 Labs turntable, not the RS Labs one. (In fact, RS Labs does not make a turntable, so far as I can tell.)

John, I owned a Hyperspace. Great turntable. As I recall, David Fletcher's crude instruction sheet advised placing the edge of the pulley practically touching the edge of the platter, a few mm's apart. When I later read about belt creep, I understood better why he did so. A Walker Audio Motor Controller made a profound improvement in the sound of my Hyperspace, much to my amazement.